Choosing the right insulation and having it properly installed in your building is a critical factor for saving money, improving comfort and contributing to sustainability. Whether constructing a new facility or retrofitting an existing building, adding insulation can effectively combat heat transfer and reduce condensation. Some buildings have additional challenges.
Choosing and installing insulation for metal buildings
Metal buildings are unique, primarily because steel is an excellent conductor (metal has essentially no R-value or thermal resistance) of heat, and can store and radiate unwanted heat in summer conditions. In winter conditions, a cold metal roof or purlin can lead to condensation and premature building envelope failure. Ideally, a metal building includes a continuous insulation system and vapor retarder to reduce the potential for condensation.
ASTM defines a vapor retarder as a material or system that adequately retards water vapor transmission under specified conditions. The permeance of a Class 1 water vapor retarder for construction typically is less than 0.1 perm. A perm rating is a measure of the diffusion of water vapor through a material. Vapor diffusion accounts for only a small amount of the total moisture in a building. Therefore, other means should be utilized to reduce water vapor migration due to air infiltration.
A vapor retarder slows the rate of water vapor diffusion, but does not completely prevent its movement. Building occupants, certain appliances and plumbing equipment generate moisture that is carried in the air as vapor. As water vapor moves from a warm interior through construction materials to a cooler surface, the water vapor may condense as liquid water that can damage the building. It is for this reason vapor retarders are installed in buildings. The accepted design practice is to provide a water vapor retarder on the exposed surface below roof insulation in a metal building. This is to prevent condensation on the bottom side of the roof panels. Ideally, this would also include the bottom of the purlins as well.
The use of reflective insulation in metal buildings has been used for more than two decades. One of the most common products is a reflective bubble insulation (RBI) which is an insulated facing (R-1 to R-2) using low-emittance surface(s). The RBI will provide added thermal resistance to the region between the roof panels and the interior air.
This added resistance comes from the material’s thermal resistance and an increase in the air film resistance when the surface emittance is changed from a high value to the low value provided by the RBI. RBI-type products can be draped over the purlins or installed below the purlins. A combination of both insulation above and below the roof purlins provides additional benefits, and is often the only option in retrofitting metal buildings with insulation. Regardless, attention to detail during installation is important to ensure a continuous vapor retarder in the building envelope.
Covering the bottom of the metal purlin with even a small amount of insulation and a vapor retarder can make a big difference in the performance of both new and existing buildings. Retrofits can be challenging. In one case in a hot-humid southern climate, national building products distributor, Service Partners, Allen, Va., needed an insulation solution for one of its warehouses in Miami. “The main issue with our warehouse,” says Neile Thomas, branch manager, “was there was no insulation and temperatures in south Florida are daunting especially during peak summer months.”
The patented Fi-Foil RetroShield System is installed on the bottom of the purlin with a simple clip and pin device. The RetroShield System also requires that the insulation be seamed on the face of the purlins and includes a handy integrated tape tab on the insulation. The combination provides more ensurance that a good seal occurs at the seam of the insulating materials.
“As an insulation supplier, we had our choice of products,” explains Thomas. “We chose RetroShield System by Fi-Foil [in Auburndale, Fla.,] for a cost-effective solution, and also for the appearance, ease of installation and performance characteristics.” The Service Partners warehouse in Miami also faced the challenge of finding a cost-effective solution to fit around skylights and still provide the maximum benefit possible. Even in cases where warehouses remain open most of the day, using reflective insulation can reduce the radiant heat flow down to improve comfort and provide a better work environment for employees.
Bill Lippy is a past president of the Reflective Insulation Manufacturers Association International (RIMA), and president and CEO of Fi-Foil Co. in Auburndale, Fla. To learn more, visit www.fifoil.comor call (800) 448-3401.